116 PARKER— THE RETINAL IMAGE. 



but it presents all the appearances of a simpler positively phototropic 

 animal. Its transformation is most perfect and complete. Here, 

 then, the influences that cover over and obscure the fundamental 

 tropisms have been removed and the animal is reduced to that state 

 which in a way was probably characteristic of its remote ancestry. 

 Thus by a simple operation a highly complex vertebrate may be 

 reduced to a simple tropic animal. 



If this outline represents the true course of events, it follows that 

 vertebrates react in ways other than tropic in consequence of their 

 enriched sensory fields, whose details are relatively enormous as com- 

 pared with those of the simpler animals. This is especially true of 

 the retinal fields. Such enriched sensory relations have induced in 

 these complex animals the development of a vastly intricate central 

 nervous organ, and on these two elements, the complex field and the 

 intricate center, are based the possibilities of the sensations, mem- 

 ories, volitions, and other like activities that give diversity to our 

 performances as compared with those of the simpler animals. 

 Though vertebrates show little of the primitive tropic responses, the 

 insects afford interesting examples of balanced forms of behavior in 

 which, though the tropism is clearly discernible, the higher type of 

 response, the response to detail or what may be called the singular 

 response, is clearly visible. 



References. 



DoLLEY, W. F. 1916. Reactions to light in Vanessa antiopa, with Special 

 References to Circus Movements. Jour. Exp. Zool., Vol. 20, pp. 357-420. 



V. Frisch, K. 1914. Der Farbensinn und Formensinn der Biene. Zool. 

 Jahrb., Abt. Zool., Bd. 35, pp. 1-182. 



Garrey, W. E. 1918. Light and the Muscle Tonus of Insects. Jour. Gen. 

 Physiol, VoL I, pp. 101-125. 



Hooker, D. 191 i. Certain Reactions to Color in the young Loggerhead 

 Turtle. Papers Tortugas Lab., Carnegie Inst., Vol. 3, pp. 69-76. 



Loeb, J. 1918. Forced Movements, Tropisms, and Animal Conduct. Phila- 

 delphia and London, 209 pp. 



MiNNicH, D. E. 1919. The Photic Reaction of the Honey-bee, Apis mei- 

 lifera L. Jour. Exp. Zool., Vol. 29, pp. 343-425. 

 Zoological Laboratory, 

 Harvard University. 



